This report describes experiments designed to assess and illustrate the effectiveness of a new method for the measurement of cerebral interstitial pO2 in conscious rodents. It is based on the use of low frequency electron paramagnetic resonance (EPR) spectroscopy with lithium phthalocyanine as the oxygen sensitiveprobe. Magnetic resonance imaging was used to document placement of the probe in the brain, and to assess potential cerebral changes associated with the placement. The technique provided accurate and reproducible measurements of localized pO2 in the brains of conscious rodents under a variety of physiological conditions and for time periods of at least 2 weeks. Using this approach we quantitated the depressing effects on cerebral pO2 of three representative anesthetics, isoflurane, ketamine/xylazine, and sodium pentobarbital. The effects of changing the content of oxygen in the breathing gas was investigated and found to change the cerebral pO2. In experiments with gerbils, crystals of lithiumphthalocyanine were implanted in each side of the brain and using a one-dimensi onal magnetic field gradient, simultaneous measurement of pO2 values from normal and ischemic (ischemia induced by unilateral ligation of a carotid artery) hemispheres of the brain were obtained. These results demonstrate that EPR oximetry with lithium phthalocyanine is a versatile and useful method in the measurement of cerebral pO2 under various physiological and pathophysiological conditions.